File descriptors are a Unix construct for tracking open "files". In Unix
though, everything is
a file. For example, "files" could be a physical file, a virtual file
(e.g., /proc/loadavg), or network sockets. Elasticsearch requires
lots of file descriptors (e.g., every shard is composed of multiple
segments and other files, plus connections to other nodes, etc.). This
bootstrap check is enforced on OS X and Linux. To pass the file
descriptor check, you might have to configure file descriptors.